Formulations with active functional additives for 3d printing of preceramic polymers, and methods of 3d-printing the formulations

ABSTRACT

This invention provides resin formulations which may be used for 3D printing and pyrolyzing to produce a ceramic matrix composite. The resin formulations contain a solid-phase filler, to provide high thermal stability and mechanical strength (e.g., fracture toughness) in the final ceramic material. The invention provides direct, free-form 3D printing of a preceramic polymer loaded with a solid-phase filler, followed by converting the preceramic polymer to a 3D-printed ceramic matrix composite with potentially complex 3D shapes or in the form of large parts. Other variations provide active solid-phase functional additives as solid-phase fillers, to perform or enhance at least one chemical, physical, mechanical, or electrical function within the ceramic structure as it is being formed as well as in the final structure. Solid-phase functional additives actively improve the final ceramic structure through one or more changes actively induced by the additives during pyrolysis or other thermal treatment.

PRIORITY DATA

This patent application is a non-provisional application with priorityto U.S. Provisional Patent App. No. 62/428,203, filed Nov. 30, 2016;U.S. Provisional Patent App. No. 62/428,207, filed Nov. 30, 2016; andU.S. Provisional Patent App. No. 62/428,213, filed Nov. 30, 2016, andU.S. Provisional Patent App. No. 62/556,388, filed Sep. 9, 2017, each ofwhich is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to monomer formulations suitablefor making preceramic polymers, which can be converted into ceramicmatrix composites and other ceramic structures.

BACKGROUND OF THE INVENTION

Ceramic matrix composite (CMC) materials overcome many disadvantages ofconventional ceramics, such as brittle failure, low fracture toughness,and limited thermal shock resistance. Applications of ceramic matrixcomposites include those requiring reliability at high temperatures(beyond the capability of metals or polymers) and resistance tocorrosion and wear.

There is also high commercial demand for additively manufactured(3D-printed) ceramics in fields including industrial filtration (moltenmetal filters, flow separators); metal processing (castingmolds/blanks); implantable dental and medical devices; and semiconductorprocessing. Additive manufacturing of ceramic materials is also ofinterest for propulsion components, thermal protection systems, porousburners, microelectromechanical systems, and electronic devicepackaging, for example.

No mature method for 3D printing ceramic matrix composites exists.Currently, CMC materials are limited to manual lay-up, molding, orthermoforming. There are also known techniques for sintering ceramicparticles or using ceramic particles printed in a binder, both of whichtypically produce porous ceramics which have lower strength than theparent material. Ceramic structures are typically sintered as compactedporous materials, severely limiting the manufacturable geometries.

Formulations have been described for creating ceramic materials, whichcan be printed (additively manufactured) with various methods such asstereolithography techniques and laser sintering. These are typicallyunreinforced ceramics that do not contain a second phase and suffer fromlow fracture toughness. These methods are described in Zocca et al.,“Additive Manufacturing of Ceramics: Issues, Potentialities, andOpportunities”, J. Am. Ceram. Soc., 98 [7] 1983-2001 (2015).

In addition, formulations which can create 1D or 2D ceramics, or verysmall 3D structures, have been described. See U.S. Pat. No. 4,816,497issued Mar. 28, 1989 to Lutz et al.; U.S. Pat. No. 5,698,485 issued Dec.16, 1997 to Bruck et al.; U.S. Pat. No. 6,573,020 issued Jun. 3, 2003 toHanemann et al.; U.S. Pat. No. 7,582,685 issued Sep. 1, 2009 to Arney etal.; and U.S. Patent App. Pub. No. US2006/0069176A1 published Mar. 30,2006 to Bowman et al.

In comparison with metals and polymers, ceramics are difficult toprocess, particularly into complex shapes. Because they cannot be castor machined easily, ceramics are typically consolidated from powders bysintering or deposited in thin films. Flaws, such as porosity andinhomogeneity introduced during processing, govern the strength becausethe flaws initiate cracks, and—in contrast to metals—brittle ceramicshave little ability to resist fracture. This processing challenge haslimited the ability to take advantage of ceramics' impressiveproperties, including high-temperature capability, environmentalresistance, and high strength. Recent advances in additive manufacturinghave led to a multitude of different techniques, but all additivemanufacturing techniques developed for ceramic materials only processunreinforced ceramics and not ceramic matrix composites. Only a few ofthe commercially available three-dimensional (3D) printing systems offerprinting of ceramics, either by selective curing of a photosensitiveresin that contains ceramic particles, selective deposition of a liquidbinder agent onto ceramic particles (binder jetting), or selectivefusion of a powder bed with a laser. All these techniques are limited byslow fabrication rates, and in many cases, a time-consuming binderremoval process. By starting with powders that need to be consolidatedto a dense part, it is an almost insurmountable challenge to addreinforcement and create ceramic matrix composites without fusing orreacting the matrix and the second phase, losing reinforcing capability.Furthermore, many additive processes introduce large thermal gradientsthat tend to cause cracks in ceramics. Pores, cracks, andinhomogeneities are often responsible for the low strength and poorreliability of additively manufactured ceramic parts.

Preceramic polymers are a class of polymers which allow, via a thermaltreatment, a conversion of a polymer part to a ceramic material.Typically, these preceramic polymers contain silicon (Si) in themolecular backbone, with the resulting material containing Si. There area wide variety of known preceramic polymers. Examples includepolysilazanes, borazine-modified hydridopolysilazanes, polysilanes,polycarbosilanes, silicone resins, polyvinylborazine, polyborazylene,and decaborane-based polymers. These preceramic polymers have been usedto form specific polymer-based structures that can be subsequentlyheat-treated (pyrolyzed or sintered) to create near net-shape ceramicstructures.

A stereolithography technique provides a method to build a 3D polymermicrostructure in a layer-by-layer process. This process usuallyinvolves a platform (e.g., substrate) that is lowered into aphotomonomer bath in discrete steps. At each layer, a laser is used toscan over the area of the photomonomer that is to be cured (i.e.,polymerized) for that particular layer. Once the layer is cured, theplatform is lowered by a specific amount, determined by the processingparameters and desired feature/surface resolution, and the process isrepeated until the complete 3D structure is created. One example of sucha stereolithography technique is disclosed in U.S. Pat. No. 4,575,330issued Mar. 11, 1986 to Hull et al.

Modifications to the above-described stereolithography technique havebeen developed to improve the polymer resolution by using laser opticsand special resin formulations. Also, modifications have been made todecrease the fabrication time of the 3D polymer structure by using adynamic pattern generator to cure an entire layer at once. One exampleof such a modification is disclosed in Bertsch et al.,“Microstereo-lithography: A Review,” Materials Research SocietySymposium Proceedings, Vol. 758, 2003. Another advancement to thestandard stereolithography technique includes a two-photonpolymerization process, as disclosed in Sun et al., “Two-PhotonPolymerization And 3D Lithographic Microfabrication,” Advances inPolymer Science, Vol. 170, 169-273, 2004.

There exists a need for creating ceramic parts of various sizes through3D printing, for engineering and other applications. Lower-coststructures that are lightweight, strong, and stiff, but can withstand ahigh-temperature oxidizing environment, are sought. There is a desirefor a method of direct 3D printing of ceramics reinforced withparticles, whiskers, or fibers, also known as ceramic matrix compositestructures.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and then further described in detail below.

Some variations provide a 3D-printing composition comprising:

-   -   (a) from about 10 vol % to about 99 vol % of one or more        preceramic, UV-curable, silicon-containing monomers in a liquid        phase; and    -   (b) from about 1 vol % to about 70 vol % of solid-phase fillers,        wherein the solid-phase fillers may be selected from the group        consisting of SiOC, SiCN, SiC, SiCBN, SiOCN, SiAlON, Si₃N₄,        SiO₂, silicate glasses, Al₂O₃, ZrO₂, TiO₂, carbon, TiC, ZrC,        HfC, Y₃Al₅O₁₂, B₄C, BN, TiN, ZrN, AlN, and combinations thereof.

In some embodiments, the preceramic, UV-curable, silicon-containingmonomers are selected from the group consisting of silazanes, siloxanes,silanes, carbosilanes, and combinations, analogues, or derivativesthereof.

In some embodiments, the solid-phase fillers are in the form of fibers,whiskers, nanotubes, nanorods, flat platelets, microparticles withaverage diameter from 1 micron to 100 microns, nanoparticles withaverage diameter from 1 nanometer to 1000 nanometers, or combinationsthereof.

In certain embodiments, the solid-phase fillers are in the form offibers with average length from 1 micron to 100 microns and with averagediameter that is less than 10% of the average length, and wherein thesolid-phase fillers are selected from the subgroup consisting of Si₃N₄,Al₂O₃, SiO₂, BN, Y₃Al₅O₁₂, ZrO₂, and combinations thereof.

In certain embodiments, the solid-phase fillers are coated with aninterfacial coating that includes a material such as (but not limitedto) BN, C, AlN, or combinations thereof. That is, solid-phase fillersmay be in the form of particles, whiskers, fibers, or other particles,in which surfaces are coated with an interfacial coating.

At least some of the solid-phase fillers may be chemically bonded to thesilicon-containing monomers. At least some of the solid-phase fillersmay contain a surface treatment that increases the compatibility,solubility, and/or bonding reactivity of the solid-phase fillers withthe silicon-containing monomers. For example, the solid-phase fillersmay contain one or more surface-functional groups selected from thegroup consisting of silane, methoxy silane, ethoxy silane, vinyl,ethynyl, vinyl ether, vinyl ester, vinyl amide, vinyl triazine, vinylisocyanurate, acrylate, methacrylate, diene, triene, mercapto, thiol,oxirane, oxetane, and combinations, analogues, or derivatives thereof.

In some embodiments, the composition further comprises a reactive ornon-reactive surfactant and/or a reactive or non-reactive wetting agent.

At least some of the solid-phase fillers may be coated with aUV-reflective material, such as a UV-reflective material selected fromthe group consisting of Al, Ni, Sn, Ag, Rh, Au, and combinations oralloys thereof.

At least some of the solid-phase fillers may be coated with a protectivematerial that inhibits degradation of the solid-phase fillers duringhigh-temperature pyrolysis.

At least some of the solid-phase fillers may be coated with asacrificial material that selectively degrades, thereby inhibitingdegradation of the solid-phase fillers during high-temperaturepyrolysis.

Other variations of the invention provide a 3D-printing compositioncomprising:

-   -   (a) from about 10 vol % to about 99 vol % of one or more        preceramic monomers in a liquid phase; and    -   (b) from about 1 vol % to about 70 vol % of solid-phase fillers,        wherein the solid-phase fillers contain one or more        surface-functional groups selected from the group consisting of        silane, methoxy silane, ethoxy silane, vinyl, ethynyl, vinyl        ether, vinyl ester, vinyl amide, vinyl triazine, vinyl        isocyanurate, acrylate, methacrylate, diene, triene, mercapto,        thiol, oxirane, oxetane, and combinations, analogues, or        derivatives thereof.

In some embodiments, the preceramic monomers are UV-curable monomersselected from the group consisting of silazanes, siloxanes, silanes, andcombinations, analogues, or derivatives thereof.

In some embodiments, the preceramic monomers are non-UV-curablemonomers, wherein the surface-functional groups react with thepreceramic monomers when exposed to UV radiation.

The solid-phase fillers may be selected from the group consisting ofSiOC, SiCN, SiC, SiCBN, SiOCN, SiAlON, Si₃N₄, SiO₂, silicate glasses,Al₂O₃, ZrO₂, TiO₂, carbon, TiC, ZrC, HfC, Y₃Al₅O₁₂, B₄C, BN, TiN, ZrN,AlN, and combinations thereof.

In certain embodiments, at least some of the surface-functional groupsare attached to silane compounds that are in turn attached to thesolid-phase fillers.

In some embodiments, the surface-functional groups, when exposed to UVradiation or visible light, react with the preceramic monomers.

The solid-phase fillers may be in the form of fibers, whiskers,nanotubes, nanorods, flat platelets, microparticles with averagediameter from 1 micron to 100 microns, nanoparticles with averagediameter from 1 nanometer to 1000 nanometers, or combinations thereof.Alternatively, or additionally, the solid-phase fillers may be in theform of fibers with average length from 1 micron to 100 microns and withaverage diameter that is less than 10% of the average length, whereinthe solid-phase fillers are selected from the subgroup consisting ofSi₃N₄, Al₂O₃, SiO₂, BN, Y₃Al₅O₁₂, ZrO₂, and combinations thereof.

In some embodiments, least some of the solid-phase fillers are coatedwith a protective material that inhibits degradation of the solid-phasefillers during high-temperature pyrolysis, wherein the protectivematerial is disposed between the surface-functional groups and thesurface of the solid-phase fillers.

In some embodiments, at least some of the solid-phase fillers are coatedwith a sacrificial material that selectively degrades, therebyinhibiting degradation of the solid-phase fillers duringhigh-temperature pyrolysis, wherein the sacrificial material is disposedbetween the surface-functional groups and the surface of the solid-phasefillers.

In certain embodiments, the solid-phase fillers are in the form offibers with average length from 1 micron to 100 microns and with averagediameter that is less than 10% of the average length, wherein thesolid-phase fillers are coated with an interfacial coating selected fromBN, C, AlN, or combinations thereof, and wherein the interfacial coatingis disposed between the surface-functional groups and the surface of thesolid-phase fillers.

Some embodiments provide a ceramic matrix composite comprising apyrolyzed form of a 3D-printed, UV-cured composition(s) as describedabove.

Other variations provide a method of making a ceramic matrix composite,the method comprising:

-   -   (i) obtaining a 3D-printing composition;    -   (ii) 3D-printing and polymerizing the 3D-printing composition to        generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic matrix composite,    -   wherein the 3D-printing composition comprises:    -   (a) from about 10 vol % to about 99.9 vol % of one or more        preceramic, UV-curable, silicon-containing monomers; and    -   (b) from about 1 vol % to about 70 vol % of solid-phase fillers,        wherein the solid-phase fillers are selected from the group        consisting of SiOC, SiO₂, SiCN, SiC, SiCBN, SiOCN, SiAlON,        Si₃N₄, SiO₂, silicate glasses, Al₂O₃, ZrO₂, TiO₂, carbon, TiC,        ZrC, HfC, Y₃Al₅O₁₂, B₄C, BN, TiN, ZrN, AlN, and combinations        thereof.

Other variations provide a method of making a ceramic matrix composite,the method comprising:

-   -   (i) obtaining a 3D-printing composition;    -   (ii) 3D-printing and polymerizing the 3D-printing composition to        generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic matrix composite,    -   wherein the 3D-printing composition comprises:    -   (a) from about 10 vol % to about 99.9 vol % of one or more        preceramic monomers; and    -   (b) from about 1 vol % to about 70 vol % of solid-phase fillers,        wherein the solid-phase fillers contain one or more        surface-functional groups selected from the group consisting of        silane, vinyl, ethynyl, vinyl ether, vinyl ester, vinyl amide,        vinyl triazine, vinyl isocyanurate, acrylate, methacrylate,        diene, triene, and combinations, analogues, or derivatives        thereof.

In other variations of the present invention, a preceramic monomerformulation is provided for 3D-printing and (typically UV-initiated)cationic polymerization, the monomer formulation comprising:

-   -   (a) a monomer molecule containing (i) non-carbon atoms, such as        (but not limited to) atoms selected from the group consisting of        Si, B, Al, Ti, Zn, P, S, Ge, and combinations thereof, and (ii)        two or more functional groups selected from the group consisting        of aliphatic ether, cyclic ether, vinyl ether, epoxide,        cycloaliphatic epoxide, oxetane, and combinations, analogues, or        derivatives thereof;    -   (b) a cationic photoinitiator or photoacid generator that may        (in some embodiments) generate a Brønsted acid when exposed to        light; and    -   (c) a 3D-printing resolution agent selected from the group        consisting of UV or visible-light absorbers, fluorescent        molecules, optical brighteners, and combinations thereof.

The formulation may contain more than one type of the monomer molecule.In some embodiments, at least 10% or at least 40% (on an atom basis) ofthe non-carbon atoms is Si.

In some embodiments, the cationic photoinitiator or photoacid generatoris present in a concentration from about 0.001 wt % to about 10 wt % inthe formulation. The formulation may include a photoacid generator thatcleaves when exposed to light to generate a Brønsted acid. Theformulation may alternatively or additionally include an ionic photoacidgenerator or a non-ionic photoacid generator.

In some embodiments, the formulation comprises or further comprises athermal cationic initiator.

In some embodiments, the formulation comprises a cationic photoinitiatorthat is active at a first wavelength, as well as a radiation-triggerBrønsted acid generator that is active at a second wavelengthsubstantially different from the first wavelength (i.e., the cationicphotoinitiator active wavelength).

The 3D-printing resolution agent may be present in a concentration fromabout 0.001 wt % to about 10 wt % in the formulation. The 3D-printingresolution agent may be selected from the group consisting of2-(2-hydroxyphenyl)-benzotriazole, 2-hydroxyphenyl-benzophenones,2-hydroxyphenyl-s-triazines, thiophenediyl)bis(5-tert-butylbenzoxazole),2,2′-(1,2-ethenediyl)bis(4,1-phenylene)bisbenzoxazole, and combinationsthereof.

In some embodiments, the formulation further comprises a UV sensitizerthat forms an excited state under UV light absorption.

In preferred embodiments, the formulation further comprises from about0.1 vol % or about 1 vol % to about 70 vol % of solid-phase fillers.

Other variations provide a preceramic monomer formulation for3D-printing and (typically UV-initiated) free-radical polymerization,the monomer formulation comprising:

-   -   (a) a monomer molecule containing (i) non-carbon atoms that are        optionally selected from the group consisting of Si, B, Al, Ti,        Zn, P, S, Ge, N, O, and combinations thereof, and (ii) two or        more C═X double bonds, two or more C≡X triple bonds, or at least        one C═X double bond and at least one C≡X triple bond, wherein X        is selected from C, S, N, O, or a combination thereof;    -   (b) a photoinitiator that generates free radicals when exposed        to light;    -   (c) a free-radical inhibitor; and    -   (d) a 3D-printing resolution agent selected from the group        consisting of UV absorbers, fluorescent molecules, optical        brighteners, and combinations thereof.

In some embodiments, at least one of the C═X double bonds or the C≡Xtriple bonds is located at a terminal position of the monomer molecule.

The monomer molecule may include two or more functional groups selectedfrom the group consisting of vinyl, ethynyl, vinyl ether, vinyl ester,vinyl amide, vinyl triazine, vinyl isocyanurate, acrylate, methacrylate,diene, triene, and combinations, analogues, or derivatives thereof.Alternatively, or additionally, the monomer molecule may contain analkyl group, an ester group, an amine group, a hydroxyl group, or acombination thereof.

In some embodiments, the non-carbon atoms are selected from the groupconsisting of Si, B, Al, Ti, Zn, P, S, Ge, and combinations thereof. Inthese or other embodiments, X is selected from C, S, or a combinationthereof.

In some embodiments, at least 10 wt % of the monomer molecule isinorganic. In some embodiments, at least 10% (on an atom basis) of thenon-carbon atoms is Si.

In some embodiments, the photoinitiator is present in a concentrationfrom about 0.001 wt % to about 10 wt % in the formulation. In certainembodiments, the photoinitiator generates free radicals byintramolecular bond cleavage or intermolecular hydrogen abstraction whenexposed to light having a wavelength from about 200 nm to about 500 nm.The photoinitiator may be selected from the group consisting of2,2-dimethoxy-2-phenylacetophenone, 2-hydroxy-2-methylpropiophenone,camphorquinone, bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide,benzophenone, benzoyl peroxide, and combinations thereof.

In some embodiments, the formulation further comprises a thermalfree-radical initiator, such as one selected from the group consistingof benzoyl peroxide, dicumyl peroxide, 2,2′-azobisisobutyronitrile, andcombinations thereof, for example.

In certain embodiments, the formulation further includes aradiation-trigger free-radical initiator active at a wavelengthsubstantially different from the photoinitiator active wavelength (i.e.the primary 3D-printing wavelength).

The free-radical inhibitor may present in a concentration from about0.001 wt % to about 10 wt % in the formulation. The free-radicalinhibitor may be selected from the group consisting of hydroquinone,methylhydroquinone, ethylhydroquinone, methoxyhydroquinone,ethoxyhydroquinone, monomethylether hydroquinone, propylhydroquinone,propoxyhydroquinone, tert-butylhydroquinone, n-butylhydroquinone, andcombinations thereof, for example.

The 3D-printing resolution agent may be present in a concentration fromabout 0.001 wt % to about 10 wt % in the formulation. The 3D-printingresolution agent may be selected from the group consisting of2-(2-hydroxyphenyl)-benzotriazole, 2-hydroxyphenyl-benzophenones,2-hydroxyphenyl-s-triazines, thiophenediyl)bis(5-tert-butylbenzoxazole),2,2′-(1,2-ethenediyl)bis(4,1-phenylene)bisbenzoxazole, and combinationsthereof.

Preferred formulations herein further include about 1 vol % to about 70vol % of solid-phase fillers.

Some embodiments provide a ceramic structure comprising a pyrolyzed formof a 3D-printed, UV-cured composition as described.

Other variations provide a method of making a ceramic structure, themethod comprising:

-   -   (i) obtaining a preceramic monomer formulation;    -   (ii) 3D-printing and polymerizing the preceramic resin        formulation to generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic structure,    -   wherein the preceramic resin formulation comprises:    -   (a) a monomer molecule containing (i) non-carbon atoms selected        from the group consisting of Si, B, Al, Ti, Zn, P, S, Ge, and        combinations thereof, and (ii) two or more functional groups        selected from the group consisting of aliphatic ethers, cyclic        ether, vinyl ether, epoxide, cycloaliphatic epoxide, oxetane,        and combinations, analogues, or derivatives thereof;    -   (b) a cationic photoinitiator or photoacid generator that may        generate a Brønsted acid when exposed to light; and    -   (c) a 3D-printing resolution agent selected from the group        consisting of UV absorbers, fluorescent molecules, optical        brighteners, and combinations thereof.

Other variations provide a method of making a ceramic structure, themethod comprising:

-   -   (i) obtaining a preceramic monomer formulation;    -   (ii) 3D-printing and polymerizing the preceramic resin        formulation to generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic structure,    -   wherein the preceramic resin formulation comprises:    -   (a) a monomer molecule containing (i) non-carbon atoms selected        from the group consisting of Si, B, Al, Ti, Zn, P, S, Ge, N, O,        and combinations thereof, and (ii) two or more C═X double bonds,        two or more C≡X triple bonds, or at least one C═X double bond        and at least one C≡X triple bond, wherein X is selected from C,        S, N, O, or a combination thereof;    -   (b) a photoinitiator that generates free radicals when exposed        to light;    -   (c) a free-radical inhibitor; and    -   (d) a 3D-printing resolution agent selected from the group        consisting of UV absorbers, fluorescent molecules, optical        brighteners, and combinations thereof.

Still other variations of the present invention provide a 3D-printingcomposition comprising:

-   -   (a) from about 10 vol % to about 99 vol % of one or more        preceramic, UV-curable monomers (typically in a liquid phase);        and    -   (b) from about 1 vol % or about 1 vol % to about 70 vol % of        solid-phase functional additives, wherein the functional        additives have at least one average dimension from about 5        nanometers to about 50 microns, and wherein the functional        additives are characterized in that when heated, the functional        additives are reactive with the monomers to cause an increase in        volume of the composition.

In some embodiments, when the functional additives are heated, they arereactive with the monomers. In these or other embodiments, thefunctional additives are characterized in that when heated under aheating atmosphere, the functional additives are reactive with one ormore gases (e.g., O₂ and/or N₂) contained in the heating atmosphere.

In some embodiments, the preceramic, UV-curable monomers are selectedfrom unsaturated ethers, vinyls, acrylates, methacrylates, cyclic ethers(epoxies or oxetanes), thiols, or a combination thereof. In otherembodiments, the preceramic, UV-curable monomers are selected fromsilazanes, siloxanes, silanes, carbosilanes, or a combination thereof.

In some embodiments, the preceramic, UV-curable monomers contain (i)non-carbon atoms selected from the group consisting of Si, B, Al, Ti,Zn, P, S, Ge, and combinations thereof, and (ii) two or more functionalgroups selected from the group consisting of aliphatic ethers, cyclicether, vinyl ether, epoxide, cycloaliphatic epoxide, oxetane, andcombinations, analogues, or derivatives thereof.

In some embodiments, the preceramic, UV-curable monomers contain (i)non-carbon atoms selected from the group consisting of Si, B, Al, Ti,Zn, P, S, Ge, N, O, and combinations thereof, and (ii) two or more C═Xdouble bonds, two or more C≡X triple bonds, or at least one C═X doublebond and at least one C≡X triple bond, wherein X is selected from C, S,N, O, or a combination thereof.

The functional additives may be selected from the group consisting ofscandium, yttrium, titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt,nickel, zinc, boron, aluminum, gallium, silicon, germanium, phosphorus,and combinations, alloys, oxides, carbides, or nitrides thereof. Inthese or other embodiments, the functional additives may be selectedfrom the group consisting of titanium silicide, chromium silicide,magnesium silicide, zirconium silicide, molybdenum silicide, andcombinations or silicates thereof. In certain embodiments, thefunctional additives are selected from the group consisting of aluminum,titanium, zirconium, titanium silicide, chromium silicide, magnesiumsilicide, zirconium silicide, and combinations thereof.

Other variations provide a method of making a ceramic structure, themethod comprising:

-   -   (i) obtaining a 3D-printing composition comprising (a) from        about 10 vol % to about 99 vol % of one or more preceramic,        UV-curable monomers; and (b) from about 1 vol % to about 70 vol        % of solid-phase functional additives, wherein the functional        additives have at least one average dimension from about 5        nanometers to about 50 microns, and wherein the functional        additives are characterized in that when heated, the functional        additives are reactive with the monomers to cause an increase in        volume of the composition;    -   (ii) 3D-printing and polymerizing the 3D-printing composition to        generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic structure, wherein during the thermally treating, the        functional additives react with the monomers to cause an        increase in volume, or reduce volumetric shrinkage, of the        ceramic structure compared to a ceramic structure that does not        contain the functional additives.

Other variations provide a 3D-printing composition comprising:

-   -   (a) from about 10 vol % to about 99 vol % of one or more        preceramic, UV-curable monomers, wherein at least some of the        preceramic, UV-curable monomers contain thiol groups; and    -   (b) from about 1 vol % to about 70 vol % of solid-phase        functional additives, wherein the functional additives have at        least one average dimension from about 5 nanometers to about 50        microns, and wherein the functional additives are characterized        in that when heated, the functional additives reactively bind        with sulfur contained in the thiol groups of the monomers.

The functional additives may be selected from the group consisting ofscandium, yttrium, titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt,nickel, zinc, boron, aluminum, gallium, silicon, germanium, phosphorus,and combinations, alloys, oxides, carbides, or nitrides thereof. Incertain embodiments, the functional additives are selected from thegroup consisting of titanium, zirconium, hafnium, silicon, aluminum,chromium, niobium, chromium silicide, titanium silicide, andcombinations thereof.

A ceramic structure is provided, comprising a pyrolyzed form of a3D-printed, UV-cured composition as disclosed. In some embodiments, theceramic structure contains sulfur in a concentration from about 0.01 wt% to about 30 wt % on an elemental sulfur basis. For example, theceramic structure may contain sulfur in a concentration from about 0.1wt % to about 10 wt % on an elemental sulfur basis.

Other variations provide a method of making a ceramic structure, themethod comprising:

-   -   (i) obtaining a 3D-printing composition comprising (a) from        about 10 vol % to about 99 vol % of one or more preceramic,        UV-curable monomers, wherein at least some of the preceramic,        UV-curable monomers contain thiol groups; and (b) from about 1        vol % to about 70 vol % of solid-phase functional additives,        wherein the functional additives have at least one average        dimension from about 5 nanometers to about 50 microns, and        wherein the functional additives are characterized in that when        heated, the functional additives reactively bind with sulfur        contained in the thiol groups of the monomers;    -   (ii) 3D-printing and polymerizing the 3D-printing composition to        generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic structure, wherein during the thermally treating, the        functional additives reactively bind with the sulfur contained        in the thiol groups.

Other variations provide a 3D-printing composition comprising:

-   -   (a) from about 10 vol % to about 99 vol % of one or more        preceramic, UV-curable monomers, preferably in a liquid phase;        and    -   (b) from about 1 vol % to about 70 vol % of solid-phase        functional additives, wherein the functional additives have at        least one average dimension from about 5 nanometers to about 50        microns, and wherein the functional additives are characterized        in that when heated, the functional additives catalyze        nucleation and/or crystallization associated with conversion of        the monomers into a solid ceramic phase.

In some embodiments, the preceramic, UV-curable monomers are selectedfrom unsaturated ethers, vinyls, acrylates, methacrylates, cyclic ethers(epoxies or oxetanes), thiols, or a combination thereof. In otherembodiments, the preceramic, UV-curable monomers are selected fromsilazanes, siloxanes, silanes, or a combination thereof.

The preceramic, UV-curable monomers may contain (i) non-carbon atomsselected from the group consisting of Si, B, Al, Ti, Zn, P, S, Ge, andcombinations thereof, and (ii) two or more functional groups selectedfrom the group consisting of aliphatic ethers, cyclic ether, vinylether, epoxide, cycloaliphatic epoxide, oxetane, and combinations,analogues, or derivatives thereof.

In some embodiments, the preceramic, UV-curable monomers contain (i)non-carbon atoms selected from the group consisting of Si, B, Al, Ti,Zn, P, S, Ge, N, O, and combinations thereof, and (ii) two or more C═Xdouble bonds, two or more C≡X triple bonds, or at least one C═X doublebond and at least one C≡X triple bond, wherein X is selected from C, S,N, O, or a combination thereof.

In certain embodiments, the preceramic, UV-curable monomers consist ofor contain carbosilanes, wherein the functional additives consist of orcontain silicon carbide (optionally as β-SiC).

In certain embodiments, the preceramic, UV-curable monomers consist ofor contain silazanes and/or polysesquiazanes, wherein the functionaladditives consist of or contain silicon nitride (optionally as α-Si₃N₄and/or β-Si₃N₄) and/or contain silicon carbide (optionally as β-SiC), ora combination thereof.

A ceramic structure is provided, comprising a pyrolyzed form of a3D-printed, UV-cured composition as disclosed.

Other variations provide a method of making a ceramic structure, themethod comprising:

-   -   (i) obtaining a 3D-printing composition comprising (a) from        about 10 vol % to about 99 vol % of one or more preceramic,        UV-curable monomers; and (b) from about 1 vol % to about 70 vol        % of solid-phase functional additives, wherein the functional        additives have at least one average dimension from about 5        nanometers to about 50 microns, and wherein the functional        additives are characterized in that when heated, the functional        additives catalyze nucleation and/or crystallization associated        with conversion of the monomers into a solid ceramic phase;    -   (ii) 3D-printing and polymerizing the 3D-printing composition to        generate a preceramic polymer; and    -   (iii) thermally treating the preceramic polymer to produce a        ceramic structure, wherein during the thermally treating, the        functional additives catalyze nucleation and/or crystallization        of a solid ceramic phase in the ceramic structure.

In various embodiments of the invention, the functional additives are inthe form of fibers, whiskers, nanotubes, nanorods, flat platelets,microparticles with average diameter from 1 micron to 100 microns,nanoparticles with average diameter from 1 nanometer to 1000 nanometers,or combinations thereof. In certain embodiments, the functionaladditives are in the form of fibers with average length from 1 micron to100 microns and with average diameter that is less than 10% of theaverage length.

In various embodiments, the functional additives are coated with one ormore compounds or chemical groups that polymerize or crosslink themonomer when exposed to UV radiation and/or heat. The compounds orchemical groups may be selected from the group consisting of unsaturatedethers, vinyls, acrylates, methacrylates, cyclic ethers (epoxies oroxetanes), and combinations thereof.

In various embodiments, the functional additives are covalently bondedto one or more compounds or chemical groups selected from the groupconsisting of unsaturated ethers, vinyls, acrylates, methacrylates,cyclic ethers, epoxies, oxetanes, amines, hydroxyls, isocyanates,hydrides, thiols, and combinations thereof.

In various embodiments, at least some of the functional additivescontain a surface treatment that increases the compatibility,solubility, and/or bonding reactivity of the functional additives withthe monomers. For example, at least some of the functional additives maybe surface-treated with silane compounds.

In various embodiments, the composition further comprises a reactive ornon-reactive surfactant and/or a reactive or non-reactive wetting agent.

In various embodiments, the functional additives are at least partiallytransparent to UV light. Alternatively, or additionally, the functionaladditives may be at least partially reflective of UV light.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is an optical microscope image of SiC solid-phase fillersdispersed in a preceramic silicone monomer (scale bar 500 μm), inExample 1.

FIG. 1B is an optical microscope image of SiC solid-phase fillersdispersed in a preceramic silicone monomer (scale bar 200 μm), inExample 1.

FIG. 2 is a photograph of a preceramic polymer (right-hand side) and apyrolyzed ceramic part (left-hand side), in Example 2.

FIG. 3 is a graph of thermogravimetric analysis for the pyrolysis of aUV-cured preceramic polymer into a pyrolyzed ceramic material, measuringthe loss of sample mass over time as the pyrolysis temperatureincreases, in Example 2.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The compositions (also referred to as formulations), structures,systems, and methods of the present invention will be described indetail by reference to various non-limiting embodiments.

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention. These and otherembodiments, features, and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following detailed description of the invention inconjunction with the accompanying drawings.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as is commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Unless otherwise indicated, all numbers expressing conditions,concentrations, dimensions, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending at least upona specific analytical technique.

The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the named claimelements are essential, but other claim elements may be added and stillform a construct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step,or ingredient not specified in the claim. When the phrase “consists of”(or variations thereof) appears in a clause of the body of a claim,rather than immediately following the preamble, it limits only theelement set forth in that clause; other elements are not excluded fromthe claim as a whole. As used herein, the phrase “consisting essentiallyof” limits the scope of a claim to the specified elements or methodsteps, plus those that do not materially affect the basis and novelcharacteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter may include the use ofeither of the other two terms. Thus in some embodiments not otherwiseexplicitly recited, any instance of “comprising” may be replaced by“consisting of” or, alternatively, by “consisting essentially of.”

Variations of this invention provide resin formulations which may beused for 3D printing (e.g., by stereolithography) of an intermediatestructure followed by thermally treating (e.g., by firing or pyrolyzing)to convert the 3D intermediate structure into a 3D ceramic structure.The monomers and polymeric systems can be printed into potentiallycomplex 3D shapes with high thermal stability and mechanical strength.

“Preceramic” in this disclosure simply refers to the capability to beultimately converted to a ceramic material. It is noted that thedisclosed preceramic resin formulations are precursors to preceramicpolymers, which themselves are precursors to ceramic materials. Asintended herein, a “resin” means a composition capable of beingpolymerized or cured, further polymerized or cured, or crosslinked.Resins may include monomers, oligomers, prepolymers, or mixturesthereof.

The extremely high melting point of many ceramics poses a challenge toadditive manufacturing to make a 3D part, as compared with metals andpolymers. Ceramics cannot be cast or machined easily. By contrast, thepresent methods enable geometrical flexibility. As described herein,preceramic resins that are cured with ultraviolet (UV) light in astereolithography 3D printer or through a patterned mask, for example,form 1D, 2D, or 3D polymer structures that can have complex shape andcellular architecture. These polymer structures can then be thermallyconverted to the corresponding 1D, 2D, or 3D ceramic part, preferablywith low shrinkage, or at least uniform shrinkage.

Some variations of the invention are premised on direct, free-form 3Dprinting of a preceramic polymer loaded with a solid-phase filler,followed by converting the preceramic polymer to a 3D-printed ceramicmatrix composite. The monomers and polymeric systems are selected withspecific properties so that they can be printed using 3D-printingmethods including stereolithography into complex 3D shapes. Someembodiments provide free-form ceramic matrix composite parts containingUV-cured, 3D-printed (e.g., stereolithographically), solid-filledpreceramic Si-containing polymer resins, or related monomerformulations. As used herein, “polymer resin” means monomer, oligomer,prepolymer, or other molecule that is converted to a polymer.

The preceramic monomer formulations are designed to allow the ceramicstructures to be formed with preferably high thermal stability (such aschemical and physical stability at temperatures greater than 1500° C.)and good mechanical strength (including stiffness, flexural strength,hardness, and/or fracture toughness). The solid solid-phase filler,among other benefits, can improve mechanical properties, especially thefracture toughness of the (otherwise) brittle ceramic material.

The invention in various embodiments applies to additively manufacturedcomponents, such as to reduce part count, scrap, or non-recurringengineering. Some embodiments apply to high-wear or high-temperatureapplications that would necessitate ceramic materials. Specificapplications of interest include, for example, propulsion structures(vanes, impellors, nacelles, and thrusters), control surfaces (fins andleading edges), hypersonic structures (thermal protection systems andheat shields), high-wear components (brakes, clutches, and rotors),catalyst support structures, pump components, filters, brakes, andclutches.

This disclosure describes resin formulation families and methods for 3Dprinting of preceramic polymer parts with solid solid-phase fillers, andthen firing or pyrolyzing the part into a ceramic. The ceramic materialsmay be prepared from a wide variety of preceramic monomer formulationsthat can be used in UV-cure-based 3D printing. Stereolithography, laserrastering, digital light processing, liquid crystal device projection,or other techniques may be employed to 3D print the monomerformulations.

The preceramic monomer formulations are loaded with a dissimilar solidmaterial, or multiple solid materials, as solid-phase fillers to formpolymer composite parts that can be directly converted to ceramic matrixcomposites (CMCs) via pyrolysis or other thermal treatment. Thesolid-phase fillers may include fibers, whiskers, platelets, particles,nanoparticles, nanotubes, or other forms of materials which can at leastpartially survive the pyrolysis conditions. Exemplary solid-phasefillers include, but are not limited to, carbides, oxides, nitrides, orcarbon (such as diamond). Certain exemplary solid-phase fillers include,but are not limited to, SiC, C, Al₂O₃, SiO₂, mullite (Al₂O₃—SiO₂),Si₃N₄, SiAlON, BN, and/or YAG (Y₃Al₅O₁₂).

Following pyrolysis, the ceramic material comprises interconnectedthree-dimensional ceramic matrix materials such as, but not limited to,silicon oxycarbide (SiOC), silicon carbide (SiC), silicon nitride(Si₃N₄), silicon oxynitride (SiON), silicon oxycarbonitride (SiOCN),silicon carbonitride (SiCN), silicon boronitride (SiBN), silicon boroncarbonitride (SiBCN), and/or boron nitride (BN).

In some variations, a monomer formulation is a mixture of a liquidpreceramic monomer resin and a solid solid-phase filler. The liquidresin is preferably UV-curable to enable definition of three-dimensionalshapes via a 3D-printing process.

Note that in this disclosure, all references to “UV,” “UV-curable,”“UV-cure-based” and the like shall include reference not only toultraviolet radiation but also other electromagnetic radiation bandsthat can be effective in various embodiments, including microwaveradiation, terahertz radiation, infrared radiation, visible radiation(light), ultraviolet radiation, and X-rays.

In some embodiments, the UV-curable monomer formulation comprises afirst molecule containing two or more unsaturated C═X double bonds orC≡X triple bonds (or at least one C═X double bond and at least one C≡Xtriple bond). X is selected from C, S, O, N, or a combination thereof,so these functional groups include C═C double bond, C≡C triple bond,C═S, and C≡N. Any H atoms involved in these functional groups may besubstituted with other atoms such as F or Cl, or side groups such asalkyl, ester, amine, hydroxyl, or CN. The first molecule may containdifferent combinations of these different unsaturated bonds. Typicalunsaturated bonds are C═C double bonds at the terminal position of themolecules, in which three hydrogen atoms are bonded to carbon atoms onthe C═C bonds (i.e., R—HC═CH₂ where R is the remainder of the firstmolecule). Other examples of these functional groups include vinyl,ethynyl, vinyl ether, vinyl ester, vinyl amide, vinyl triazine, vinylisocyanurate, acrylate, methacrylate, diene, triene, or a mixturethereof.

The first molecule also contains at least one non-carbon atom in themain chain or side chains of the first molecule. Examples of non-carbonatoms that may be used include, but are not limited to, Si, B, Al, Ti,Zn, O, N, P, S, Ge, and combinations thereof. The non-carbon atoms maybe a part of cyclic or acyclic groups or structures within the firstmolecule. The non-carbon atoms are preferably not merely singlenon-carbon atoms ionically bonded at the end(s) of the first molecule.In some embodiments, when X is O, the non-carbon atom is not O; or whenX is N, the non-carbon atom is not N.

Examples of the first molecules include, but are not limited to,trivinylborazine; 2,4,6-trimethyl-2,4,6-trivinylcyclotrisilazane;1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasilazane;1,3,5-trivinyl-1,3,5-trimethylcyclosiloxane;1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane;2,2,4,4,6,6-hexakisallyloxyl-triazatriphosphinine; tetraallyloxysilane;vinyl-terminated polydimethylsiloxane; tetravinylsilane;vinyl-terminated polydimethylsiloxane-ethylene copolymer;divinyldimethylsilane; 1,2-divinyltetramethyldisilane;1,4-bis(vinyldimethylsilyl)benzene; vinylmethylsiloxane homopolymer;methacryloxypropyl-terminated polydimethylsiloxane; boronvinyldimethylsiloxide; vinylmethylsiloxane-dimethylsiloxane copolymer,trimethylsiloxy-terminated homopolymer;vinylethoxysiloxane-propylethoxysiloxane copolymer;vinyltrimethoxysilane; trivinylmethylsilane; diallyldimethylsilane;1,3,5-trisilacyclohexane; B,B′B″-trithynyl-N,N′N″-trimethylborazine;B,B′B″-triethynylborazine; vinylmethoxysiloxane,acryloxypropyl(methylsiloxane) homopolymer; or a combination thereof.

The first molecule, when present, may be up to about 100 wt % of themonomer formulation. In various embodiments, the first molecule is about5, 10, 20, 30, 40, 50, 60, 70, 80, 90, or 95 wt % of the monomerformulation.

In some embodiments, the UV-curable monomer formulation comprises asecond molecule with a structure R—Y—H, wherein Y═O, S, N, orcombinations thereof. The molecules R—Y—H can provide two or more YHgroups for polymerization, and can be part of cyclic or acyclicstructures. Typical YH groups are SH groups, e.g. thiol or mercaptogroups. The R groups can be organic groups such as alkyl groups, estergroups, amine groups, or hydroxyl groups, or inorganicnon-carbon-containing atoms or groups. Examples of inorganic non-carbonatoms or groups in the second molecule include, but are not limited to,Si, B, Al, Ti, Zn, P, Ge, S, O, N, or combinations thereof. The reactionrate varies depending on the different molecules utilized. In somepreferred embodiments, a thiol is employed with at least half of themain chain made of inorganic atoms, such as silicon. Other atoms in themain chain may include oxygen, nitrogen, and/or carbon.

The second molecule, when present, may be up to about 97 wt % of themonomer formulation. In various embodiments, the second molecule isabout 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, or 90 wt % of the monomerformulation. The second molecule may be present whether or not the firstmolecule is present.

Exemplary second molecule include, but are not limited to,pentaerythritol tetrakis(3-mercaptopropionate);trimethylolpropanetris(2-mercaptoacetate); trimethylolpropanetris(3-mercaptopropionate);tetrakis(dimethyl-3-mercaptopropylsiloxy)silane;tetrakis(dimethyl-2-mercaptoacetate siloxy)silane;(mercaptopropyl)methylsiloxane-dimethylsiloxane copolymer;(mercaptopropyl)methylsiloxane homopolymer; pentaerythritoltetrakis(2-mercaptoacetate); or a combination thereof.

In some embodiments, the UV-curable monomer formulation comprises athird molecule with a structure R—Y, wherein Y is selected from analiphatic ether, a cyclic ether, a vinyl ether, an epoxy, acycloaliphatic epoxy, an oxcetane group, or a combination thereof. The Rgroups may be selected from organic groups such as alkyl groups, estergroups, amine groups, or hydroxyl groups, or inorganic non-carboncontaining atoms or groups. Examples of inorganic non-carbon atoms orgroups in the second molecule include, but are not limited to, Si, B,Al, Ti, Zn, P, Ge, S, O, N, or combinations thereof. The inorganicnon-carbon atoms or groups may be a part of cyclic or acyclicstructures.

Exemplary third molecules include, but are not limited to,epoxy-functional dimethylpolysiloxane and/or epoxycyclohexylethylmethylsiloxane/dimethylsiloxane. These monomers can be any portion ofthe monomer formulation.

In particular, the third molecule, when present, may be up to about 100wt % of the monomer formulation. In various embodiments, the thirdmolecule is about 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, or 95 wt % ofthe monomer formulation. The third molecule may be present whether ornot the first or second molecules are present.

In some embodiments, the UV-curable monomer formulation comprises aphotoinitiator that generates free radicals under light exposure byintramolecular bond cleavage or intermolecular hydrogen abstraction. Thephotoinitiator may be active in the presence of light having awavelength from about 200 nm to about 500 nm, for example.Photoinitiators may be used when the polymerization is, or includes,free-radical polymerization. Photoinitiators may be used to initiatepolymerization when exposed to other wavelengths, such as in the visiblespectrum. In certain embodiments, light exposure is produced from lighthaving one or more wavelengths selected from about 200 nm to about 700nm, such as about 250, 300, 350, 400, 500, or 600 nm.

Different photoinitiators will generally result in different reactionrates for polymerization. A combination of different types ofphotoinitiators may be used in the polymerization process. More than onephotoinitiator may be included to allow multi-wavelength curing, forexample.

Examples of photoinitiators include, but are not limited to,2,2-dimethoxy-2-phenylacetophenone; 2-hydroxy-2-methylpropiophenone;camphorquinone; bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide;benzophenone; benzoyl peroxide; thioxanones; dicumyl peroxide;2,2′-azobisisobutyronitrile; camphorquinone; oxygen; nitrogen dioxide;or a combination thereof.

The photoinitiator, when present, may be up to about 10 wt % of themonomer formulation. In various embodiments, the photoinitiator is about0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 2, 5, or 10 wt % of the monomerformulation.

In some embodiments, the UV-curable monomer formulation comprises afree-radical inhibitor added in a sufficient amount to the monomerformulation to inhibit unwanted polymerization of regions outside thedesired printing area. A free-radical inhibitor can improve resolutionto the desired part in embodiments that employee free-radicalpolymerization. A free-radical inhibitor can also deter shadow curing,which is normally not desired. Additionally, a free-radical inhibitorcan improve long-term stability of the formulation and keep reactionkinetic parameters constant over time.

Exemplary free-radical inhibitors include, but are not limited to,hydroquinone, methylhydroquinone, ethylhydroquinone,methoxyhydroquinone, ethoxyhydroquinone, monomethylether hydroquinone,propylhydroquinone, propoxyhydroquinone, tert-butylhydroquinone,n-butylhydroquinone, or a combination thereof. When present, thefree-radical inhibitor may be up to about 5 wt % of the monomerformulation, such as about 0.001, 0.005, 0.01, 0.05, 0.1, 0.2, 0.5, 1,or 2 wt % of the monomer formulation.

Optionally the formulation further includes a radiation-triggerfree-radical initiator that is active at a wavelength substantiallydifferent from the photoinitiator. When the preceramic resin formulationincludes a thermal free-radical initiator, optionally the formulationfurther includes a radiation-trigger free-radical initiator.

In some embodiments, the UV-curable monomer formulation comprises afree-radical thermal initiator that generates free radicals underelevated temperature conditions. The addition of a free-radical thermalinitiator allows for multiple-mechanism curing in the formulation, i.e.,both UV curing and thermal curing, or allows for a differentpolymerization reaction rate. One or a combination of different types ofthermal initiators may be used in the polymerization process.

A thermal initiator may be used to crosslink unreacted vinyl groupsremaining which have not reacted with the thiol group or to react thevinyl group with other available functional groups such as methyl orhydro groups on the first or second molecule, creating a second type ofreaction mechanism. A thermal post-cure after 3D printing may be done,such as by heating the polymer structure up to 300° C.

Exemplary free-radical thermal initiators include, but are not limitedto, benzoyl peroxide, dicumyl peroxide, 2,2′-azobisisobutyronitrile, ora combination thereof. When present, the free-radical thermal initiatormay be up to about 10 wt % of the monomer formulation, such as about0.001, 0.01, 0.1, 1, 2, or 5 wt % of the monomer formulation.

In some embodiments, the UV-curable monomer formulation comprises acationic photoinitiator or photoacid generator, such as (but not limitedto) sulphonium, iodonium, and/or ferrocenium cation paired with anon-nucleophilic anion. For example, the UV-curable resin may contain asalt which under light exposure creates acids (e.g., Brønsted acids) bycleavage of the sulphonium, iodonium, and/or ferrocenium cation of theonium salt, paired with a proton donor. Cationic photoinitiators aretypically active under light wavelengths from 200 nm to 350 nm.Initiators that are active at lower or higher wavelengths are alsoapplicable to these monomer formulations. Cationic photoinitiators orionic photoacid generators may be used when the polymerization is, orincludes, cationic polymerization. Different cationic photoinitiators orphotoacid generators will generally result in different reaction ratesfor polymerization. A combination of different types of cationicphotoinitiators and/or photoacid generators (including ionic andnon-ionic photoacid generators) may be used in the polymerizationprocess.

Exemplary cationic photoinitiators or photoacid generators include, butare not limited to, sulfonium, iodonium, and ferrocenium salts;cyclopentacienylcumene-iron hexafluoro phosphate; diphenyliodoniumphosphate; triarylsulfonium hexafluoroantimonate; or a combinationthereof.

The cationic photoinitiator or photoacid generator, when present, may beup to about 10 wt % of the monomer formulation. In various embodiments,the cationic photoinitiator or photoacid generator is about 0.001,0.005, 0.01, 0.05, 0.1, 0.5, 1, 2, 5, or 10 wt % of the monomerformulation.

In certain embodiments, the UV-curable monomer formulation comprises ahydrogen donor that may be used to assist in the generation of aBrønsted acid in the cation or in acceleration of anionic photoinitiatorreactions, for example. Exemplary hydrogen donors include, but are notlimited to, tertiary amines, alcohols, ethers, esters, water, or acombination thereof. When present, the hydrogen donor may be up to about2 wt % of the monomer formulation, such as about 0.001, 0.005, 0.01,0.05, 0.1, 0.5, 1, or 1.5 wt % of the monomer formulation.

In some embodiments, the UV-curable monomer formulation comprises a UVsensitizer that may be used to enable the long-UV-wavelength reaction ofUV systems with photoinitiators which typically absorb at lowerwavelengths. This is typically the case with cationic photoinitiators,which are generally limited to absorption up to about 325-375 nm, forexample. UV sensitizers interact with UV light at higher wavelengths,generally into the 375-425 nm range, and then interact with thephotoinitiator to create either free radicals and/or Brønsted acids. AUV sensitizer forms an excited triplet state under UV light absorption,and then via electron or energy transfer, reacts with a photoinitiatorto generate free radicals and/or Brønsted acids. This initiatesphotopolymerizaton.

UV sensitizers may be selected from dibutoxyantracene,diethoxyanthracene, 1-chloro-4-propoxythioxanthone,2-isopropylthioxanthone, 4-isopropylthioxanthone, or a combinationthereof, for example. When present, the UV sensitizer may be up to about5 wt % of the monomer formulation, such as about 0.001, 0.005, 0.01,0.05, 0.1, 0.5, 1, 2, 3, or 4 wt % of the monomer formulation.

In some embodiments, including those utilizing free-radicalpolymerization, cationic polymerization, or both of these, theUV-curable monomer formulation comprises one or more 3D-printingresolution agents selected from UV absorbers, fluorescents, opticalbrighteners, or a combination thereof.

A “3D-printing resolution agent” is a compound that improves printquality and resolution by containing the curing to a desired region ofthe laser or light exposure. In certain embodiments, the 3D-printingresolution agent functions by absorbing light (e.g., UV or visiblelight) at a desired wavelength and converting the energy either intothermal energy or radiation at a higher wavelength. The use of3D-printing resolution agents improves print quality and resolution bycontaining the curing by the laser or light exposure to the desiredregion laterally and/or vertically in the print bath.

Exemplary 3D-printing resolution agents include, but are not limited to,2-(2-hydroxyphenyl)-benzotriazole; 2-hydroxyphenyl-benzophenones;2-hydroxyphenyl-s-triazines;2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole);ethenediyl)bis(4,1-phenylene)bisbenzoxazole; or a combination thereof.When present, the 3D-printing resolution agent may be up to about 10 wt% of the monomer formulation, such as about 0.001, 0.01, 0.1, 0.5, 1, 2,3, 4, 5, 6, 7, 8, or 9 wt % of the monomer formulation.

Some variations provide a preceramic resin formulation comprising:

-   -   (a) a first molecule comprising two or more C═X double bonds,        two or more C≡X triple bonds, or at least one C═X double bond        and at least one C≡X triple bond, wherein X is selected from the        group consisting of C, S, N, O, and combinations thereof, and        wherein the first molecule further comprises at least one        non-carbon atom selected from the group consisting of Si, B, Al,        Ti, Zn, P, Ge, S, N, O, and combinations thereof;    -   (b) optionally a second molecule comprising R—Y—H, wherein R is        an organic group or an inorganic group, and wherein Y is        selected from the group consisting of S, N, O, and combinations        thereof (Y is not yttrium in this specification);    -   (c) a photoinitiator and optionally a thermal free-radical        initiator;    -   (d) a free-radical inhibitor; and    -   (e) a 3D-printing resolution agent.

In some embodiments, the first molecule is present from about 3 wt % toabout 97 wt % of the formulation, such as about 4, 5, 10, 15, 20, 25,30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95 wt %, forexample.

In some embodiments, the first molecule contains two or more C═X doublebonds, and at least one of these double bonds is located at a terminalposition of the first molecule. In some embodiments, the first moleculecontains two or more C≡X triple bonds, and at least one of these triplebonds is located at a terminal position of the first molecule. In someembodiments, the first molecule contains at least one C≡X double bondand at least one C≡X triple bond, and the C═X double bond is located ata terminal position, or the C≡X triple bond is located at a terminalposition, or both of the C═X double bond and the C≡X triple bond arelocated at (different) terminal positions within the first molecule.Note that a molecule may contain more than two terminal positions, whenthere is branching present.

In the first molecule, the non-carbon atom may be present in the mainchain, in side chains, or in both of these.

The first molecule may include one or more functional groups selectedfrom the group consisting of vinyl, ethynyl, vinyl ether, vinyl ester,vinyl amide, vinyl triazine, vinyl isocyanurate, acrylate, methacrylate,diene, triene, and analogues thereof. In some embodiments, the firstmolecule includes two or more of such functional groups. An “analogue”herein means that the functional group has similar chemical and reactiveproperties, with respect to the polymerization of the preceramic resinformulation.

In some embodiments in which the second molecule is included in thepreceramic resin formulation, the second molecule is present from about0.1 wt % to about 97 wt % of the formulation, such as about 0.2, 0.5, 1,2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80,85, 90, or 95 wt %, for example.

The second molecule may include one or more functional groups selectedfrom the group consisting of thiol, alkyl, ester, amine, hydroxyl, andfunctional analogs thereof. Alternatively, or additionally, the secondmolecule may be chemically contained within one or more functionalgroups selected from the group consisting of thiol, alkyl, ester, amine,hydroxyl, and analogues thereof.

When the second molecule is present, the R group may be, or include, aninorganic group containing an element selected from the group consistingof Si, B, Al, Ti, Zn, P, Ge, S, N, O, and combinations thereof.

In some embodiments, at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%,45%, or 50% (mole percent) of the R group is inorganic, i.e. not carbon.In certain embodiments, at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%,45%, or 50% (mole percent) of the R group is specifically Si.

In the second molecule, the R group may be present in the main chain, inside chains, or in both of these. The non-carbon atom of the R group,when it is inorganic, may be the same as or different than thenon-carbon atom in the first molecule.

The weight ratio of the second molecule to the first molecule may varyfrom about 0 to about 32, such as about 0.5, 1, 2, 3, 5, 10, 15, 20, 25,or 30. In some embodiments, the weight ratio of second molecule to firstmolecule is dependent on the ratio of thiol to vinyl. For example, incertain embodiments there is at least one thiol functional groupavailable per vinyl group.

As noted earlier, some variations of the invention employ a combinationof free-radical polymerization and cationic polymerization. In someembodiments, a preceramic monomer formulation compatible withstereolithography or UV-cure 3D printing leverages both cationic andfree-radical polymerization mechanisms, wherein the formulationcomprises:

-   -   (a) a first molecule comprising two or more C═X double bonds or        C≡X triple bonds, wherein X is selected from C and/or S, or from        C, S, O, and/or N, and wherein the first molecule further        comprises at least one non-carbon atom in the main chain or side        chains selected from the group consisting of Si, B, Al, Ti, Zn,        P, S, Ge, and combinations thereof;    -   (b) a second molecule comprising two or more thiol or mercapto        (SH) groups, wherein the second molecule further comprises at        least one non-carbon atom in the main chain or side chains        selected from the group consisting of Si, B, Al, Ti, Zn, P, S        Ge, and combinations thereof (preferably at least 10 wt %, more        preferably at least 40 wt %, of the non-carbon atoms being        inorganic, such as Si), and wherein the second molecule may be a        part of an alkyl group, ester group, amine group, or hydroxy        group;    -   (c) a third molecule comprising two or more functional groups        selected from aliphatic ether, cyclic ether, vinyl ether, epoxy,        cycloaliphatic epoxy, oxcetane, or a combination thereof,        wherein the third molecule further contains at least one        non-carbon atom in the main chain or side chains selected from        the group consisting of Si, B, Al, Ti, Zn, P, S, Ge, and        combinations thereof;    -   (d) a photoinitiator that generates free radicals by        intramolecular bond cleavage and/or intermolecular hydrogen        abstraction under exposure to light having a wavelength from        about 200 nm to about 500 nm;    -   (e) a cationic photoinitiator or photoacid generator that may        generate Brønsted acids under a light exposure;    -   (f) a free-radical inhibitor, wherein the free-radical inhibitor        is added in a sufficient amount to the monomer formulation to        inhibit unwanted polymerization of regions outside the desired        exposure and deter shadow curing;    -   (g) UV absorbers, fluorescents, and/or optical brighteners added        in a sufficient amount to the monomer formulation to improve        print quality and resolution by containing the curing to the        region of the laser or light exposure to the desired region;    -   (h) optionally a UV sensitizer to enable the long-UV-wavelength        reaction of UV systems with photoinitiators that typically        absorb at lower wavelengths, forming an excited triplet state        under UV light absorption, followed by electron or energy        transfer to react with a photoinitiator to generate free        radicals and/or Brønsted acids, thereby initiating        photopolymerizaton; and    -   (i) optionally from about 0.1 vol % to about 70 vol % of one or        more solid-phase fillers as set forth herein.

In preferred embodiments, the UV-curable monomer formulation furthercomprises one or more solid-phase fillers. A “solid-phase filler” asmeant herein is a material that (a) forms at least one solid phase at25° C. and 1 atm, and (b) enhances at least one chemical, physical,mechanical, or electrical property within the UV-curable monomerformulation or a reaction product thereof. A solid-phase filler is notmerely a low-cost, diluent material (known also as extenders) but ratheran important component of some formulations disclosed herein.

The solid-phase filler may be from about 0.1 vol % or about 1 vol % toabout 70 vol % of the monomer formulation, with the majority of theremainder typically being liquid-phase UV-curable monomer.

The geometric shape of the solid-phase filler may be fibers includingshort fibers (1-100 micrometers in length) or long fibers (>100micrometers in length), whiskers, nanotubes, nanorods, flat platelets,microparticles with diameters between 1 and 100 micrometers,nanoparticles with diameters between 1 and 1000 nanometers, or acombination thereof.

To increase fracture toughness of a 3D-printed part, solid-phase fillerswith aspect ratios of at least 2 are preferred, such as fibers,whiskers, nanotubes, and nanorods. Here, “aspect ratio” is the ratio ofaverage length to average width, or in the case of an arbitrary shape,the ratio of average maximum length scale to average minimum lengthscale. The solid-phase filler aspect ratio is preferably at least 5,more preferably at least 10, in certain embodiments.

The solid-phase filler composition is preferably stable at a pyrolysistemperature of at least 800° C., so as not to disintegrate, melt, orvaporize during later conversion of a preceramic polymer to a ceramicmaterial. Note that the solid-phase filler may react at pyrolysistemperatures with other components present in the monomer formulation orits reaction products (e.g., polymer) or with furnace atmosphere gases.It is possible for a portion of the solid-phase filler to react awayinto the vapor phase, or into a liquid phase, during high-temperatureprocessing.

In certain embodiments, a solid-phase filler precursor is introduced tothe monomer formulation, wherein the precursor is in a liquid phase oris a gel, for example. The solid-phase filler precursor may then reactor undergo a phase change, such as during polymerization, to convert thesolid-phase filler precursor into a solid-phase filler.

The solid-phase filler may have a wide range of compositions. Forexample, solid-phase filler compositions may include, but are notlimited to, silicon-based ceramics such as SiOC, SiO₂, SiCN, SiC, SiCBN,SiOCN, Si₃N₄, silicate glasses, etc. Solid-phase filler compositions mayinclude non-silicon-based ceramics such as metal oxides, e.g. Al₂O₃,ZrO₂, TiO₂, or Y₃Al₅O₁₂. Solid-phase filler compositions may includecarbon-based, high-temperature materials such as carbon, graphene,diamond, and metal carbides, e.g. TiC, ZrC, HfC, or B₄C. Solid-phasefiller compositions may include nitride-based ceramics, e.g. BN, TiN,ZrN, or AlN.

Solid-phase fillers interact with UV light according to Snell's law andthe well-known Fresnel equations. These laws of physics determine thefractions of the light that are reflected, transmitted, or absorbed whenUV light passes from resin to filler. For a UV-based 3D printingprocess, it is preferred that the fillers do not absorb too much UVlight which would hinder complete UV curing of the resin. To avoidabsorption of too much UV light, a low level of solid-phase filler maybe employed, such as less than 10 vol % of relatively small (e.g., 10micron or smaller) particles. Alternatively, or additionally, asolid-phase filler that is somewhat transparent to UV light and lets UVlight pass through, may be employed. Another approach to ensure that UVlight is not excessively absorbed by the filler particles is to employparticles with a surface that reflects UV light. For example, aluminumreflects UV light well. For maximum reflection, the surface of suchparticle should be smooth. Surface treatments or coatings may be appliedto render the surface of filler particles reflective—such as a thincoating of aluminum or silver.

Preferred solid-phase filler materials, in some embodiments, are shortfibers of alumina (Al₂O₃), quartz (SiO₂), glass, silicon nitride(Si₃N₄), yttrium aluminum garnet (YAG), or boron nitride (BN) becausethese materials transmit at least some UV light. SiC or C fibers absorbtoo much UV light and therefore should to be coated with a reflectivecoating, to enable efficient 3D printing.

Depending on the chemistry and viscosity of the monomer formulation(resin), the solid-phase filler may be treated to increase itscompatibility with and wetting of the resin, the solubility anddispersion of the filler in the resin, and/or the bonding between thefiller and the resin. In some embodiments, dispersion aides may bechosen to match the isoelectric point of the solid-phase filler particleand the chemistry and functionality of the monomer resin.

Some embodiments employ surfactants with a component which bonds to thesurface of the filler and a component which solvates in the resinsystem. Surface functionality may be added to the surface of thesolid-phase filler by covalently bonding a functional group to thesurface of the filler. Examples include the use of silane surfacemodifiers with active groups that can either react with the chemistry ofthe resin or increase the wettability and dispersability in thesolid-phase filler. These include the addition of mercapto trimethoxysilane, vinyl trimethoxy silane, 3-glycidyl oxypropyl trimethoxy silane,or a combination thereof, for example. The surface may also be modifiedthrough other chemical means, such as vapor-solid reactions orliquid-solid reactions, e.g. oxidation in a furnace or acid or basetreatments.

For the 3D printing and curing of the resin, it can also be advantageousif the solid-phase filler itself is coated or surface-treated with achemical that contains a functional group that aids in polymerization orcrosslinking of the resin on UV and/or thermal exposure. Such functionalgroups include unsaturated ethers, vinyls, acrylates, methacrylates,cyclic ethers, epoxies, oxetanes, amines, hydroxyls, isocyanates,hydrides, or combinations thereof. By adding functional groups to thesurface of the solid-phase filler, fewer or even no functional groupsare necessary in the resin and the system can still be cured by UVexposure. Alternatively, or additionally, functional groups introducedto the surface of the solid-phase filler particles may enable a thermalcure after initial UV curing during 3D printing.

The solid-phase filler may be coated to protect it from environmentaldegradation during pyrolysis. Reactive species such as oxygen freeradicals, and other free radicals, may be generated during thepyrolysis. Such free radicals can react with the fillers and degradetheir properties. To mitigate this, the fillers may be coated with athin layer of a protective material such as BN or a sacrificial materialsuch as pyrolytic carbon that preferentially decomposes duringpyrolysis.

To increase fracture toughness of a 3D-printed ceramic matrix composite,a high-aspect-ratio filler, such as a fiber, may be coated with afiller/matrix interfacial coating. The purpose of this coating is toprovide a weak filler-matrix interface that prevents matrix cracks frompenetrating the fillers—thus providing damage tolerance (toughness) tothe composite. The interfacial coating is preferably chemically andmechanically stable during processing and pyrolysis. Examples ofinterfacial coatings include BN, C, AlN, or a combination thereof.

The formulations disclosed herein may be 3D printed using many differentmethods. In some variations, the formulations may be directly 3D printedand converted to free-form ceramic matrix composite structures. A3D-printed preceramic polymer material may be prepared directly frompreceramic monomer formulations, with no intervening steps beingnecessary. A 3D-printed ceramic material may then be prepared directlyfrom the 3D-printed preceramic polymer material, with no interveningsteps being necessary.

Preferred methods may include stereolithography, binder jetting, resinjetting with fiber placement, polyjetting, extrusion printing, or acombination thereof.

In stereolithography, the solid-phase filler is dispersed in the liquidresin (monomer formulation). Layers are cured from the top or bottomusing UV-laser rastering, projection micro-stereolithography, digitallight projection, or liquid crystal device projection, for example.Smaller filler sizes are preferred since the filler size often limitsthe resolution, depending on material choice.

Generally speaking, “jetting” of a material means that droplets of abuild material are selectively deposited onto a build bed to develop athree-dimensional object. Jetting can be carried out by liquiddeposition, vapor deposition, or liquid-vapor mist deposition, forexample, via spraying (such as via a nozzle in communication with amaterial under pressure), impingement (such as via a tube or pipe incommunication with a material that is pumped), or other means.

In binder jetting, a layer of the solid-phase filler is spread out andresin (monomer formulation) is jetted on selected locations and curedsuch as via UV light or thermally. This process is similar toconventional binder jetting methods, but instead of a binder, apreceramic monomer formulation is used. The solid-phase filler mayinitially be spread out on a substrate or on a region of polymer basedon the selected monomer, for example. After an initial step of binderjetting, another layer of the solid-phase filler may be spread out on a3D-printed polymer layer, followed by resin jetting and curing. Thisprocess may be repeated many times for large parts.

In resin jetting with fiber placement, solid-phase fillers in the formof long or short fibers are placed in the preferred location and alignedin the preferred direction. Subsequently, preceramic resin (monomerformulation) is jetted in selected locations and cured. The process isrepeated layer-by-layer to build a part. Resin jetting with fiberplacement enables printing of parts with high volume fraction (such as30-60 vol %) of aligned fibers, resulting in improved mechanicalproperties for the final ceramic structure (following pyrolysis).

In polyjetting, a mixture of liquid resin (monomer formulation) andsolid-phase filler is jetted and written into the desired pattern. Asthe mixture is dispensed, it is exposed to UV light such as a laser,LED, or plasma sources, and cured into a polymer. Multiple mixtures areable to be dispensed through different nozzles, allowing for more thanone type of monomer-filler mixture to be utilized simultaneously. Thisresults in tailored mechanical properties for the final ceramicstructure (following pyrolysis).

In extrusion printing, the resin and filler mixture is squeezed througha micro-nozzle, or multiple micro-nozzles, and cured via UV light. Oneadvantage is that high-aspect-ratio fillers can be aligned with theextrusion process. Alignment generally improves mechanical properties inthe aligned direction.

After a part is 3D printed using any of the above methods, or anothermethod, the part may be post-cured. An optional thermal post-cure of the3D polymer is performed after the 3D printing but prior to the pyrolysisto produce the ceramic structure. A post-cure step may be employed tocrosslink unreacted functional groups, for example. Post-curing may beaccomplished by additional UV exposure and/or a thermal post-cure atelevated temperatures (such as 60-500° C.) in an oven for about 10minutes to about 8 hours. When a thermal post-cure is to be done, it canbe beneficial to include a thermal initiator in the initial 3D-printingcomposition, to facilitate later thermal curing.

Typically, but not necessarily, a monomer formulation is conveyed(printed) to a region of interest, such as via stereolithography, binderjetting, resin jetting with fiber placement, polyjetting, or extrusionprinting, either followed by polymerization or with polymerizationtaking place simultaneously with the printing. Preferably, thepolymerizing and 3D printing steps are performed simultaneously, at adesired location (e.g., a layer) within a part. In some embodiments, thepolymerizing and 3D printing steps are performed semi-simultaneously, inwhich multiple steps are performed overall while at each step, someamount of polymerizing and some amount of 3D printing takes place. It isalso possible, in some embodiments, to first polymerize a preceramicresin formulation, followed by 3D printing of the already-madepolymer—especially when the polymer is a thermoplastic material.

In some embodiments, the curing or conversion of preceramic resinformulation to preceramic polymer includes crosslinking. A crosslink isa bond that links one polymer chain to another. Crosslink bonds can becovalent bonds or ionic bonds. When polymer chains are linked togetherby crosslinks, they lose some of their ability to move as individualpolymer chains. Crosslinks are the characteristic property ofthermosetting plastic materials. In most cases, crosslinking isirreversible, unless ionic bonds are employed in reversible crosslinks(see, for example, commonly owned U.S. patent application Ser. No.15/391,749, filed Dec. 27, 2016, which is hereby incorporated byreference herein).

In some embodiments, while a monomer is being converted to polymer, agel is formed first. Gel formation is followed by formation of a solidmaterial as the monomer conversion is further increased, to crosslinkchains together. A “gel” is a solid, jelly-like material that can haveproperties ranging from soft and weak to hard and tough. Gels exhibit noflow when in the steady-state. By weight, gels are mostly liquid, yetthey behave like solids due to a three-dimensional crosslinked networkwithin the liquid.

Some variations of the invention utilize a self-propagating polymerwaveguide, as described in commonly owned U.S. Pat. No. 7,687,132 issuedMar. 30, 2010 to Gross et al.; U.S. Pat. No. 9,341,775 issued May 17,2016 to Eckel et al.; U.S. Pat. No. 9,377,567 issued Jun. 28, 2016 toJacobsen et al.; and U.S. Pat. No. 9,528,776 issued Dec. 27, 2016 toRoper et al., which are hereby incorporated by reference herein. Withoutbeing limited by speculation or theory, it is hypothesized that initialexposure of monomer to a collimated beam can initiate microgel siteswithin the liquid monomer layer. These microgel sites have a highercrosslink density than the surrounding monomer/polymer, which leads to ahigher localized refractive index. The higher refractive index at themicrogel site may act as a lens. The focused energy from the incidentbeam leads to initial “waveguide” formation in the direction of theincident (primary) beam, where the refractive index of the waveguide ishigher than the surrounding monomer/polymer. U.S. Pat. No. 7,382,959issued Jun. 3, 2008 to Jacobsen is hereby incorporated by referenceherein for its description of mechanisms involving self-propagatingpolymer waveguide formation.

In exemplary embodiments, sufficient polymerization inhibitor and UVabsorber are added to the resin formulation to confine thepolymerization to the laser exposure point and to minimize scatter, thusmaintaining fidelity in the features of the printed part. UV light isthen scanned across the resin surface to expose a cross section andbuild up a thin slice of the part to be manufactured. Although inprinciple any geometry can be fabricated with this approach, the processcan be slow, because every thin layer has to be exposed separately.

Structures with linear features extending from the exposure surface,such as lattices and honeycombs, can be formed much more rapidly whenutilizing the self-propagating photopolymer waveguide technology.Monomers are selected to promote a change in the index of refractionupon polymerization, which causes internal reflection of the UV light,trapping it in the already-formed polymer. This exploits a self-focusingeffect that forms a polymer waveguide, tunneling the light toward thetip of the waveguide and causing it to polymerize further. There is areduced need for additives that control scatter and UV absorption. Thearchitecture of the material or structure can then be defined by apatterned mask that defines the areas exposed to a collimated UV lightsource, for example. The polymer crosslink density depends on exposureparameters and can be increased by thermal treatments or additional UVexposure. Unpolymerized resin may be recycled and reused.

The direct, near-net-shape conversion of a preceramic 3D-printed polymerto a ceramic structure may be achieved by pyrolysis or other thermaltreatment, such as (but not limited to) sintering, annealing, orcalcination. Typically, the thermal treatment is based on heating the3D-printed structure for an extended period of time (such as from 10minutes to 1 week) under various inert or reactive atmospheres.

Thermal treatment may be done for an extended period of time undervarious atmospheres, including but not limited to N₂, Ar, He, air, CO₂,CH₄, C₂H₆, C₂H₄, NH₃, or a combination thereof. Treatment pressures mayvary from about 1 atm to about 20 atm, for example. Vacuum pyrolysis mayalso be employed, in which the treatment pressure is less than 1 atm,again under various atmospheres as noted above.

The pyrolysis or other thermal treatment may include heating at aheating rate of 0.1-20° C./min from ambient temperature to an elevatedtemperature from about 500° C. to about 1500° C., such as from about800° C. to about 1100° C. These slow heating rates are preferred toenable evolving gases to escape, thereby minimizing porosity in thefinal part. When porosity is desired, higher heating rates (e.g., higherthan 20° C./min) may be employed. The pyrolysis or other thermaltreatment may also include dwelling at the elevated temperature (e.g.,950° C.) for at least 1, 5, 10, 15, 30, or 60 minutes. Followingpyrolysis, the material may be cooled at a cooling rate (magnitude) of0.1-20° C./min back to ambient temperature. In some embodiments, fastercooling (e.g., higher than 20° C./min in magnitude) is desired tofreeze-in a desired microstructure, for example.

The thermal treatment is preferably performed following polymerizationand any (optional) thermal post-cure of the 3D polymer. In certainembodiments, the thermal treatment is combined (i.e., overlaps in timeand/or temperature) with polymerization, thermal post-cure, or both. Itwill also be recognized that even when a sequential operation isintended, some amount of ceramic formation may occur prior to a plannedstep of thermal treatment, as a result of the intrinsic kinetics andthermodynamics of the reaction system.

In some embodiments, a reactive thermal treatment is performed, in whichthe gas that is initially present is reactive toward the initialpolymer, the final ceramic material, or both of these. When the gas isreactive, it may react with a component and cause it to leave thematerial. Alternatively, or additionally, the gas may react with acomponent and remain with the base material. It is also possible for thegas to react and form products, some of which depart from the materialwhile the rest remains with the material. Reactive gases may be selectedfrom O₂, O₃, air, CO, CO₂, H₂, H₂O, CH₄, SO₂, H₂S, NH₃, NO, NO₂, andN₂O, and so on. The maximum temperature for reactive thermal treatmentmay be, for example, about 300° C. to about 1500° C. The system pressuremay also be adjusted to influence the gas atmosphere.

The pyrolysis or other thermal treatment process produces a ceramic partor ceramic matrix composite which may include various ceramic materialssuch as, but not limited to, SiC, SiOC, Si₃N₄, SiON, SiCN, SiOCN, SiBN,SiBCN, BN, or a combination thereof. The composition of the ceramic partor ceramic matrix composite obviously is directly dependent on thecomposition of the starting 3D-printing monomer formulation as providedin this disclosure. When carbon is desired in the ceramic material, thefraction of carbon may be tailored, for example, by adding phenyl groupson the side chain of the polymer or by using a carbon-based crosslinkingagent such as divinyl benzene.

In some embodiments, final ceramic structures are lightweight, strong,and stiff—but can withstand a high-temperature oxidizing environment.The configuration and microstructure of the preceramic polymer determinethe composition, microstructure, and yield of the ceramic material afterthermal treatment. A high crosslink density is preferred to prevent thefragmentation and loss of low-molecular-mass species, which have notfully converted to either ceramic or escaping gases, during thermaltreatment.

During the thermal treatment, whether an inert or reactive thermaltreatment technique is employed, gases escape. Gases are formed duringthe conversion of preceramic polymer to the ceramic structure, bydecomposition reactions of the polymer, photoinitiator, free-radicalinhibitor, and/or 3D-printing resolution agent. The escaping gases orvapors may include (but are by no means limited to) CH₄, H₂, CO, CO₂,H₂O, SO₂, H₂S, CH₃S, etc.

Because various gases escape during pyrolysis or other thermaltreatment, the concentration of the solid-phase filler will typically behigher in the final ceramic material, compared to the starting3D-printing monomer formulation. This is because the solid-phase filleris typically very stable during thermal treatment and does not losemuch, if any, mass, while the polymer typically loses a significantamount of mass in pyrolysis (for example, see FIG. 3). Therefore, theconcentration of solid-phase filler may be greater than 70 vol % in thefinal ceramic structure. In various embodiments, the concentration ofsolid-phase filler is from about 0.1 vol % to about 90 vol %, such asabout 0.5, 1, 5, 10, 20, 30, 40, 50, 60, 70, or 80 vol % on the basis ofthe total ceramic structure following thermal treatment and anypost-processing (e.g., washing).

In some variations of this invention, active solid-phase functionaladditives are employed as the solid-phase fillers. By “solid-phasefunctional additives” it is meant a material that (a) forms at least onesolid phase at 25° C. and 1 atm, and (b) performs or enhances at leastone chemical, physical, mechanical, or electrical function within theceramic structure as it is being formed and in the final structure.

Note that solid-phase functional additives are distinguished from thesolid-phase fillers disclosed above. Compared to solid-phase fillers,solid-phase functional additives actively improve the final ceramicstructure through one or more changes explicitly induced by theadditives during pyrolysis or other thermal treatment, as will now bedescribed.

The solid-phase functional additives may be present from about 0.1 vol %and 70 vol % of the monomer formulation, with the majority of theremainder being liquid UV-curable resin. The solid-phase functionaladditive geometry varies. In some embodiments, the solid-phasefunctional additives are small particles with average sizes (length oreffective diameter) from 5 nanometers to 5 micrometers.

In some embodiments, the solid-phase functional additives activelyexpand in volume and counteract the shrinkage of the resin, eliminatingor reducing the overall shrinkage during conversion of the polymer toceramic. This addresses a significant shortcoming in the art.

In particular, on conversion from polymer to ceramic, typically about20-30% linear dimensional shrinkage and about 20-60% mass loss areobserved. The shrinkage facilitates cracking and distortion, and limitsthe achievable part size and tolerances. By introducing activesolid-phase functional additives that expand in volume during pyrolysis,the shrinkage of the preceramic polymer is counteracted. The overallshrinkage during conversion of the polymer to ceramic can be reduced oreven eliminated.

Note that the solid-phase functional additives are not necessarilystable (unreactive) at pyrolysis temperatures. In many case, it isdesired that the functional additives are reactive.

In particular, the solid-phase functional additives may react with thepreceramic resin directly on heat treatment. Alternatively, oradditionally, the solid-phase functional additives may react withspecies (e.g., oxygen, nitrogen or carbon) generated from decompositionof the polymer during pyrolysis. Alternatively, or additionally, thesolid-phase functional additives may react with the pyrolysis atmosphereduring the pyrolysis, for example a nitrogen, methane, or ammoniaatmosphere. To counteract the detrimental effects of shrinkage, it ispreferred that these reactions happen at the same time as the preceramicresin shrinks, or are effective to reverse the volume reduction.

Examples of solid-phase functional additives for counteracting theshrinkage of the resin include, but are not limited to, scandium,yttrium, titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, zinc,boron, aluminum, gallium, silicon, germanium, phosphorus, orcombinations thereof. Combinations of these elements such as titaniumsilicide, chromium silicide, magnesium silicide, zirconium silicide, ormolybdenum silicide may be used as the solid-phase functional additives.Preferred solid-phase functional additives in this category includealuminum, titanium, zirconium, titanium silicide, chromium silicide,magnesium silicide, and zirconium silicide.

In some embodiments, the solid-phase functional additives actively bindsulfur. For example, the solid-phase functional additives may react withsulfur from thiol groups in the resin and bind the sulfur into stablecompounds. One class of preferred UV-curable preceramic resins for 3Dprinting is based on the thiol-ene reaction (alkene hydrothiolation).The thiol groups contain sulfur which can partially remain in theceramic after pyrolysis, causing an unpleasant smell. Residual sulfurcan also corrode metals.

To mitigate the negative effects of residual sulfur, active solid-phasefunctional additives may be added that react with the sulfur and bindthe sulfur in stable compounds that are neutral in smell and neutralwith respect to corrosion of metals. Examples of solid-phase functionaladditives for binding with sulfur include, but are not limited to, Ti,Zr, Hf, Si, Al, Cr, Nb, CrSi₂, TiSi₂, or a combination thereof.Preferred sulfur gathering/scavenging solid-phase functional additivesare Ti, Zr, and Hf which react to form the stable compounds Ti₂S₃, ZrS₂,and HfS₂, respectively.

In some embodiments, the ceramic structure contains from about 0.01 wt %to about 20 wt % sulfur, such as from about 0.1 wt % to about 10 wt %sulfur. In various embodiments, the ceramic structure contains about0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 wt %sulfur. Of the sulfur that is present, from about 10 wt % to about 100wt %, such as about 20, 30, 40, 50, 60, 70, 80, or 90 wt %, may be boundto solid-phase functional additives in stable compounds containingsulfur.

In some embodiments, the solid-phase functional additives actively seedcrystallization of a preferred ceramic phase by enabling epitaxialgrowth of the preferred phase without a nucleation barrier. Afterpyrolysis of preceramic polymers, an amorphous ceramic is usuallyobtained. To increase strength and hardness, and reduce high-temperaturecreep, the amorphous ceramic material needs to then be crystallized intoa preferred ceramic phase. This is typically achieved by a long (manyhours) heat treatment at temperatures above the pyrolysis temperature,performed right after the pyrolysis or as a distinct second heattreatment.

By contrast, with appropriate solid-phase functional additives in theresin, crystallization may be facilitated by seeding crystallization.Without limitation, the mechanism may include providing a surface forepitaxial growth of the preferred phase or multiple ceramic phases.

For example, the crystallization of β-SiC in an amorphous SiC or SiCNceramic derived from a polycarbosilane-based or polysilazane-based resincan be facilitated by small (e.g., 1 nanometer to 5 microns) β-SiCcrystals. Crystallization of such a resin may be performed attemperatures between 1300° C. and 2800° C. over the course of 5 to 50hours. Similarly, the crystallization of the α phase or β phase of Si₃N₄in an amorphous Si₃N₄ or SiCN ceramic derived from a polysilazane-basedresin can be facilitated by small (e.g., 50 nanometers to 5 microns)α-Si₃N₄ or β-Si₃N₄ crystals, respectively. Other crystals may be chosento facilitate crystallization, with the typical constraint of epitaxialgrowth on one crystal facet with low lattice strain.

The ceramic structure may be characterized by at least 50% theoreticaldensity, preferably at least 75% theoretical density, and morepreferably at least 95% theoretical density. By “theoretical density” itis meant the actual density of the ceramic structure as a percentage oftheoretical density of the material itself, calculated in the absence ofporous voids. For example a ceramic structure with absolute density of2.0 g/cm³, fabricated from a base material with inherent (bulk) densityof 2.1 g/cm³, exhibits a theoretical density of 2.0/2.1=95%.

In various embodiments, the ceramic structure is characterized by atheoretical density of about (or at least about) 50%, 60%, 70%, 75%,80%, 85%, 90%, or 95%. In certain embodiments, without limitation, theceramic structure is a fully dense monolith, which means that theceramic structure has at least 99% (e.g., essentially 100%) theoreticaldensity associated with a part or continuous region of material (alsoreferred to as a “monolith”). The absolute density in g/cm³ will vary,depending on the selection of base materials; an exemplary range isabout 1 g/cm³ to about 4 g/cm³.

The overall mass loss associated with the conversion of preceramicpolymer to the ceramic structure may vary widely, such as from about 1wt % to about 90 wt %, e.g. about 5, 10, 20, 30, 40, 50, 60, 70, or 80wt %. The overall mass loss will be dictated by the starting formulation(e.g., fraction organic versus inorganic) as well as by processparameters. In principle, the lost mass may be recovered separately andused for other purposes.

Associated with mass loss may be shrinkage of the preceramic polymer asit converts to the ceramic structure. The linear shrinkage (calculatedin a single dimension, such as height of part) may be from 0% to about60%, for example. Note that the mass loss and shrinkage are notnecessarily correlated. In some embodiments with high mass loss, thereis not much (if any) shrinkage. These embodiments tend to produce higherporosity and therefore lower densities. In some embodiments with highmass loss, there is substantial shrinkage, unless certain solid-phasefillers are utilized as described above and/or solid-phase functionaladditives are utilized as described below. These embodiments tend toproduce lower porosity, or no porosity, and therefore higher densities(e.g., fully dense ceramic materials). Finally, in some embodiments,there is little mass loss but shrinkage associated with chemicalreactions taking place. These embodiments also tend to producerelatively high densities.

Despite shrinkage, if any, the bulk shape (relative geometry) of thepreceramic 3D-printed polymer may be preserved in the final ceramicstructure. That is, when shrinkage is uniform in all dimensions, thegeometrical features are retained in the part: it is a scaled-downversion, in all three dimensions. In some embodiments, shrinkage isapproximately uniform, which means the geometrical features arebasically maintained, with slight deviations. Uniform shrinkage ispossible when there is no random fragmentation during conversion of thepreceramic polymer to the ceramic structure, and when the reactions andgas escape are isotropic within the material. Note that very smallfeatures, such as at the nanoscale, may not be preserved duringotherwise uniform shrinkage.

Practically speaking, uniform shrinkage (or no shrinkage, in certainembodiments employing active functional additives) enables the formationof parts that are “net shape” or “near net shape.” “Net shape” meansthat the geometrical features are retained, so that manufactured partsallow final fabrication matching the intended design with little or nopost-processing. “Near net shape” means that the geometrical featuresare not perfectly retained but require only minimal post-processing orhand-work. Both net-shape parts and near-net-shape parts require littleor no machining, polishing, bonding, surface finishing, or assembly.

The density of the final ceramic part may vary, as explained above. Ingeneral (without limitation), absolute densities ranging from about 0.1g/cm³ to about 5 g/cm³ may be produced. A fully dense ceramic may have adensity from about 1 g/cm³ to about 4 g/cm³, for example.

The strength of the final ceramic material will vary, depending on theinitial preceramic resin formulation, as well as the processingparameters. In some embodiments, the final ceramic material ischaracterized by a Young's Modulus of at least about 200 GPa, 300 GPa,400 GPa, 500 GPa, or more, measured at 25° C. In some embodiments, thefinal ceramic material is characterized by a flexural strength of atleast about 300 GPa, 400 GPa, 500 GPa, or more, measured at 25° C. Insome embodiments, the final ceramic material is characterized by ahardness of at least about 10 GPa, 20 GPa, 30 GPa, or more, measured at25° C.

The engineering strength of a ceramic part also will depend on thegeometry—such as a microtruss produced by some embodiments employing aself-propagating polymer waveguide technique. It is noted that, forinstance, silicon oxycarbide microlattice and honeycomb cellularmaterials fabricated with the present methods exhibit higher strengththan ceramic foams of similar density.

The thermal stability of the final ceramic material will vary, dependingprimarily on the initial preceramic resin formulation, as well as theprocessing parameters. In various embodiments, the final ceramicmaterial is thermally stable at a temperature of at least 1500° C.,1600° C., 1700° C., 1800° C., 1900° C., or 2000° C. Thermal stabilitymeans at least that the ceramic material does melt at thesetemperatures, and preferably also that the ceramic material does notreact (e.g., by oxidation or reduction), undergo thermal shock, orphysically decompose (introducing defects) at these temperatures. Insome embodiments, for example, the ceramic structure is characterized bybeing stable in the presence of air at a temperature of about 1000° C.,1100° C., 1200° C., 1300° C., 1400° C., 1500° C., 1600° C., 1700° C.,1800° C., or higher.

The final ceramic structure, even when no machining, polishing, bonding,surface finishing, or assembly is required, may be subjected to coloring(e.g., with inks or dyes), stamping, or other non-functional features,if desired.

EXAMPLES Example 1 Preparation of 3D-Printing Composition for SiC/SiOCUV-Cured Ceramic Matrix Composite

A monomer mixture containing 100 parts of vinylmethoxysiloxane polymer,100 parts of (mercaptopropyl)methylsiloxane polymer, 0.5 parts of2,2-dimethyl-2-phenylacetophenone, 0.15 parts of tert-butylhydroquinone,and 0.25 parts of 2,2′-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole)(all parts by weight), is thoroughly blended to make sure the componentsare well-mixed and the mixture is a uniform system. This resin iscapable of forming a silicon oxycarbide (SiOC) ceramic phase whenpolymerized and thermally treated.

Then 25% by weight silicon carbide (SiC) powder with 50 μm particle sizeis blended and sonicated to disperse the SiC particles into the aboveresin. The SiC microparticles serve as solid-phase fillers in theSiOC-forming resin. The mixture is then ready for use as a monomerformulation in UV-cured 3D printing.

Optical microscope images of SiC solid-phase fillers dispersed in thepreceramic silicone matrix are shown in FIG. 1A (scale bar 500 μm) andFIG. 1B (scale bar 200 μm).

Example 2 Production of 3D-Printed, UV-Cured SiC/SiOC Ceramic MatrixComposite

The monomer formulation of Example 1 is 3D-printed and UV-cured,followed by thermal treatment to form a ceramic matrix composite. Bulkparts are demonstrated by curing layers at 385 nm with LED-UV to form apreceramic polymer, and then pyrolyzing the preceramic polymer at 1000°C. in inert atmosphere to form a pyrolyzed ceramic material. FIG. 2shows a photograph of the preceramic polymer (right-hand side) and thedarker pyrolyzed ceramic part (left-hand side).

FIG. 3 is a graph of thermogravimetric analysis for the pyrolysis of theUV-cured preceramic polymer into a pyrolyzed ceramic material, measuringthe loss of sample mass over time as the pyrolysis temperatureincreases.

The versatility and the applications of these preceramic resinformulations make them especially useful. A variety of applications inthe automotive and aerospace industries, among others, may benefit fromthe ability to 3D-print high-strength and high-temperature ceramicstructures which can be derived from the disclosed formulations. Theseceramic 3D parts or materials may be used for lightweight,high-temperature structural applications or for other applications thatutilize the unique microstructures, such as (but not limited to) jetengine nozzles, nose cones, catalyst support, engine components, andmicroelectromechanical systems and devices.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which are shown by wayof illustration specific exemplary embodiments of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatmodifications to the various disclosed embodiments may be made by askilled artisan.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain steps may be performed concurrently ina parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

The embodiments, variations, and figures described above should providean indication of the utility and versatility of the present invention.Other embodiments that do not provide all of the features and advantagesset forth herein may also be utilized, without departing from the spiritand scope of the present invention. Such modifications and variationsare considered to be within the scope of the invention defined by theclaims.

What is claimed is:
 1. A 3D-printing composition comprising: (a) fromabout 10 vol % to about 99 vol % of one or more preceramic, UV-curablemonomers; and (b) from about 1 vol % to about 70 vol % of solid-phasefunctional additives, wherein said functional additives have at leastone average dimension from about 5 nanometers to about 50 microns, andwherein said functional additives are characterized in that when heated,said functional additives are reactive to cause an increase in volume ofsaid composition.
 2. The composition of claim 1, wherein saidpreceramic, UV-curable monomers are selected from unsaturated ethers,vinyls, acrylates, methacrylates, cyclic ethers (epoxies or oxetanes),thiols, or a combination thereof.
 3. The composition of claim 1, whereinsaid preceramic, UV-curable monomers are selected from silazanes,siloxanes, silanes, carbosilanes, or a combination thereof.
 4. Thecomposition of claim 1, wherein said preceramic, UV-curable monomerscontain (i) non-carbon atoms selected from the group consisting of Si,B, Al, Ti, Zn, P, S, Ge, and combinations thereof, and (ii) two or morefunctional groups selected from the group consisting of aliphaticethers, cyclic ether, vinyl ether, epoxide, cycloaliphatic epoxide,oxetane, and combinations, analogues, or derivatives thereof.
 5. Thecomposition of claim 1, wherein said preceramic, UV-curable monomerscontain (i) non-carbon atoms selected from the group consisting of Si,B, Al, Ti, Zn, P, S, Ge, N, O, and combinations thereof, and (ii) two ormore C═X double bonds, two or more C≡X triple bonds, or at least one C═Xdouble bond and at least one C≡X triple bond, wherein X is selected fromC, S, N, O, or a combination thereof.
 6. The composition of claim 1,wherein said functional additives are selected from the group consistingof scandium, yttrium, titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt,nickel, zinc, boron, aluminum, gallium, silicon, germanium, phosphorus,and combinations or alloys thereof.
 7. The composition of claim 1,wherein said functional additives are selected from the group consistingof titanium silicide, chromium silicide, magnesium silicide, zirconiumsilicide, molybdenum silicide, and combinations or silicates thereof. 8.The composition of claim 1, wherein said functional additives areselected from the group consisting of aluminum, titanium, zirconium,titanium silicide, chromium silicide, magnesium silicide, zirconiumsilicide, and combinations thereof.
 9. The composition of claim 1,wherein said functional additives are characterized in that when heated,said functional additives are reactive with said monomers.
 10. Thecomposition of claim 1, wherein said functional additives arecharacterized in that when heated under a heating atmosphere, saidfunctional additives are reactive with one or more gases contained insaid heating atmosphere.
 11. A 3D-printing composition comprising: (a)from about 10 vol % to about 99 vol % of one or more preceramic,UV-curable monomers, wherein at least some of said preceramic,UV-curable monomers contain thiol groups; and (b) from about 1 vol % toabout 70 vol % of solid-phase functional additives, wherein saidfunctional additives have at least one average dimension from about 5nanometers to about 50 microns, and wherein said functional additivesare characterized in that when heated, said functional additivesreactively bind with sulfur contained in said thiol groups of saidmonomers.
 12. The composition of claim 11, wherein said functionaladditives are selected from the group consisting of scandium, yttrium,titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, manganese, iron, cobalt, nickel, zinc, boron,aluminum, gallium, silicon, germanium, phosphorus, and combinations,alloys, oxides, carbides, or nitrides thereof.
 13. The composition ofclaim 11, wherein said functional additives are selected from the groupconsisting of titanium, zirconium, hafnium, silicon, aluminum, chromium,niobium, chromium silicide, titanium silicide, and combinations thereof.14. A ceramic structure comprising a pyrolyzed form of a 3D-printed,UV-cured composition according to claim
 11. 15. The ceramic structure ofclaim 14, wherein said ceramic structure contains sulfur in aconcentration from about 0.1 wt % to about 30 wt % on an elementalsulfur basis.
 16. A 3D-printing composition comprising: (a) from about10 vol % to about 99 vol % of one or more preceramic, UV-curablemonomers; and (b) from about 1 vol % to about 70 vol % of solid-phasefunctional additives, wherein said functional additives have at leastone average dimension from about 5 nanometers to about 50 microns, andwherein said functional additives are characterized in that when heated,said functional additives catalyze nucleation and/or crystallizationassociated with conversion of said monomers into a solid ceramic phase.17. The composition of claim 16, wherein said preceramic, UV-curablemonomers are selected from unsaturated ethers, vinyls, acrylates,methacrylates, cyclic ethers (epoxies or oxetanes), thiols, or acombination thereof.
 18. The composition of claim 16, wherein saidpreceramic, UV-curable monomers are selected from silazanes, siloxanes,silanes, carbosilanes, or a combination thereof.
 19. The composition ofclaim 16, wherein said preceramic, UV-curable monomers contain (i)non-carbon atoms selected from the group consisting of Si, B, Al, Ti,Zn, P, S, Ge, and combinations thereof, and (ii) two or more functionalgroups selected from the group consisting of aliphatic ethers, cyclicether, vinyl ether, epoxide, cycloaliphatic epoxide, oxetane, andcombinations, analogues, or derivatives thereof.
 20. The composition ofclaim 16, wherein said preceramic, UV-curable monomers contain (i)non-carbon atoms selected from the group consisting of Si, B, Al, Ti,Zn, P, S, Ge, N, O, and combinations thereof, and (ii) two or more C═Xdouble bonds, two or more C≡X triple bonds, or at least one C═X doublebond and at least one C≡X triple bond, wherein X is selected from C, S,N, O, or a combination thereof.
 21. The composition of claim 16, whereinsaid preceramic, UV-curable monomers consist of or contain carbosilanes,and wherein said functional additives consist of or contain siliconcarbide (optionally as β-SiC).
 22. The composition of claim 16, whereinsaid preceramic, UV-curable monomers consist of or contain silazanesand/or polysesquiazanes, and wherein said functional additives consistof or contain silicon nitride (optionally as α-Si₃N₄ and/or β-Si₃N₄).23. The composition of any one of claim 1, 11, or 16, wherein saidfunctional additives are in the form of fibers, whiskers, nanotubes,nanorods, flat platelets, microparticles with average diameter from 1micron to 100 microns, nanoparticles with average diameter from 1nanometer to 1000 nanometers, or combinations thereof.
 24. Thecomposition of any one of claim 1, 11, or 16, wherein said functionaladditives are coated with one or more compounds or chemical groups thatpolymerize or crosslink said monomer when exposed to UV radiation and/orheat, and optionally wherein said compounds or chemical groups areselected from the group consisting of unsaturated ethers, vinyls,acrylates, methacrylates, cyclic ethers (epoxies or oxetanes), andcombinations thereof.
 25. The composition of any one of claim 1, 11, or16, wherein said functional additives are covalently bonded to one ormore compounds or chemical groups selected from the group consisting ofunsaturated ethers, vinyls, acrylates, methacrylates, cyclic ethers,epoxies, oxetanes, amines, hydroxyls, isocyanates, hydrides, andcombinations thereof.
 26. The composition of any one of claim 1, 11, or16, wherein at least some of said functional additives contain a surfacetreatment that increases the compatibility, solubility, and/or bondingreactivity of said functional additives with said monomers.
 27. Thecomposition of any one of claim 1, 11, or 16, wherein said compositionfurther comprises a reactive or non-reactive surfactant and/or areactive or non-reactive wetting agent.
 28. The composition of any oneof claim 1, 11, or 16, wherein said functional additives are at leastpartially transparent to UV light.
 29. The composition of any one ofclaim 1, 11, or 16, wherein said functional additives are at leastpartially reflective of UV light.